Speculative application

spec :: Eq a => a -> (a -> b) -> a -> bSource

spec g f a evaluates f g while forcing a, if g == a then f g is returned, otherwise f a is evaluated and returned. Furthermore, if the argument has already been evaluated, we skip the f g computation entirely. If a good guess at the value of a is available, this is one way to induce parallelism in an otherwise sequential task. However, if the guess isn't available more cheaply than the actual answer, then this saves no work and if the guess is wrong, you risk evaluating the function twice. Under high load, since 'f g' is computed via the spark queue, the speculation will be skipped and you will obtain the same answer as 'f $! a'.

The best-case timeline looks like:

 foreground: [----- a -----]
 foreground:               [-]    (check g == a)
 spark:         [----- f g -----]
 overall:    [--- spec g f a ---]

The worst-case timeline looks like:

 foreground: [----- a -----]
 foreground:               [-]               (check g == a)
 foreground:                 [---- f a ----]
 spark:         [----- f g -----]
 overall:    [-------- spec g f a ---------]

Note that, if f g takes longer than a to compute, in the HEAD release of GHC, f g will be collected and killed during garbage collection.

 foreground: [----- a -----]
 foreground:               [-]               (check g == a)
 foreground:                 [---- f a ----]
 spark:         [---- f g ----######         (#'s mark when this spark is collectable)
 overall:    [--------- spec g f a --------]

Under high load:

 foreground: [----- a -----]
 foreground:               [-]               (check g == a)
 foreground:                 [---- f a ----]
 overall:    [-------- spec g f a ---------]

Compare these to the timeline of f $! a:

 foreground: [----- a -----]
 foreground:               [---- f a ----]
 orverall:   [---------- f $! a ---------]

spec' :: Eq a => a -> (a -> b) -> a -> bSource

Unlike spec, this version does not check to see if the argument has already been evaluated. This can save a small amount of work when you know the argument will always require computation.

specBy :: (a -> a -> Bool) -> a -> (a -> b) -> a -> bSource

spec with a user defined comparison function

specBy' :: (a -> a -> Bool) -> a -> (a -> b) -> a -> bSource

spec' with a user defined comparison function

specOn :: Eq c => (a -> c) -> a -> (a -> b) -> a -> bSource

spec comparing by projection onto another type

specOn' :: Eq c => (a -> c) -> a -> (a -> b) -> a -> bSource

spec' comparing by projection onto another type

Speculative application with transactional rollback

specSTM :: Eq a => STM a -> (a -> STM b) -> a -> STM bSource

specSTM g f a evaluates fg = do g' <- g; f g', while forcing a, then if g' == a then fg is returned. Otherwise the side-effects of fg are rolled back and f a is evaluated. g is allowed to be a monadic action, so that we can kickstart the computation of a earlier.

If the argument a is already evaluated, we don't bother to perform fg at all.

If a good guess at the value of a is available, this is one way to induce parallelism in an otherwise sequential task.

However, if the guess isn't available more cheaply than the actual answer then this saves no work, and if the guess is wrong, you risk evaluating the function twice.

The best-case timeline looks like:

 foreground: [--- g >>= f ---]
 spark:          [------- a -------]
 foreground:                       [-] (compare g' == a)
 overall:    [---- specSTM g f a ----]

The worst-case timeline looks like:

 foreground: [---- g >>= f ----]
 spark:         [------- a -------]
 foreground:                      [-] (check if g' == a)
 foreground:                        [--] (rollback)
 foreground:                           [------ f a ------]
 overall:    [------------ specSTM g f a ----------------]

Under high load, specSTM degrades less gracefully than spec:

 foreground: [---- g >>= f ----]
 spark:                        [------- a -------]
 foreground:                                     [-] (check if g' == a)
 foreground:                                       [--] (rollback)
 foreground:                                          [------ f a ------]
 overall:    [--------------------specSTM g f a ------------------------]

Compare these to the timeline of f $! a:

 foreground: [------- a -------]
 foreground:                   [------ f a ------]

specSTM' :: Eq a => STM a -> (a -> STM b) -> a -> STM bSource

Unlike specSTM, specSTM' doesn't check if the argument has already been evaluated.

specOnSTM :: Eq c => (a -> STM c) -> STM a -> (a -> STM b) -> a -> STM bSource

specBySTM . on (==)

specOnSTM' :: Eq c => (a -> STM c) -> STM a -> (a -> STM b) -> a -> STM bSource

specBySTM' . on (==)

specBySTM :: (a -> a -> STM Bool) -> STM a -> (a -> STM b) -> a -> STM bSource

specSTM using a user defined comparison function

specBySTM' :: (a -> a -> STM Bool) -> STM a -> (a -> STM b) -> a -> STM bSource

specSTM' using a user defined comparison function